The present invention relates to a vertical type MOSFET and a manufacturing method thereof, in particular, in which its drift resistance is made to be low without lowering the breakdown voltage between source and drain electrodes.
FIG. 1 is a sectional view showing the structure of a conventional general use vertical type MOSFET. As shown in FIG. 1, at this vertical type MOSFET, an Nxe2x88x92type epitaxial layer 202, which is a high resistance drift layer, is formed on one surface of an N+type semiconductor substrate 201. P type base layers 203 are formed in the surface of the Nxe2x88x92type epitaxial layer 202, and N+type source layers 204 are formed in designated regions in the P type base layers 203 and a P+type base layer 205 is formed in a specified region of the P type base layers 203. Trenches 207 are formed in designated regions of the P type base layers 203 from the surface, and in each of the trenches 207, a gate insulation film 208 is formed and gate poly-silicon 209 is filled, and a trench type gate electrode 206 is formed. On a dielectric interlayer 210 covering the surface of the trench type gate electrodes 206, a source electrode 211, which is connected to the N+type source layers 204 and the P+type base layer 205, is formed. A drain electrode 212 is formed on the other surface of the N+type semiconductor substrate 201.
At this type of the vertical type MOSFET, in order to make the drift resistance low, it is desirable that the concentration of the impurity in the Nxe2x88x92type epitaxial layer 202 (high resistance drift layer) is set to be high. However, when a voltage is applied between the source electrode 211 and the drain electrode 212, a depletion layer occurs only in the vertical direction between the P type base layer 203 and the high resistance drift layer 202. Therefore, the electric field strength exceeds the critical electric field during extending the depletion layer, and the electric field centralization, which is one of the causes of lowering the breakdown voltage between the source electrode 211 and the drain electrode 212, is liable to occur. Consequently; in order to secure a certain value of the breakdown voltage between the source electrode 211 and the drain electrode 212, there is a problem that the concentration of the impurity in the high resistance drift layer 202 cannot be made to be a value being more than a specified value.
In order to solve this problem, there are conventional vertical type MOSFETs in which the drift resistance is lowered and the breakdown voltage between the source electrode and the drain electrode is made to be high. For example, as a first conventional technology, Japanese Patent Application Laid-Open No. 2001-119022 discloses a semiconductor device and a manufacturing method thereof. FIG. 2 is a sectional view showing the structure of the vertical type MOSFET at the first conventional technology. In FIG. 2, each of the parts being equivalent to one of the parts shown in FIG. 1 has the same reference number, therefore the same explanation is omitted. This vertical type MOSFET has planar type gate electrodes and the gate electrodes are not the trench type gate electrodes.
As shown in FIG. 2, plural Pxe2x88x92type layers 213 are layered in the high resistance drift layer 202 at the positions right under the P type base layer 203 in the vertical direction (in the depth direction). That is, PN junctions being in parallel to the high resistance drift layer 202 are formed. By this structure, at the time when a voltage is applied between the source electrode 211 and the drain electrode 212 in a state that the vertical type MOSFET is off, a depletion layer occurs not only in the vertical direction between the P type base layer 203 and the high resistance drift layer 202 but also in the horizontal direction between the plural Pxe2x88x92type layers 213 and the high resistance drift layer 202. With this structure, compared with a vertical type MOSFET in which the plural Pxe2x88x92type layers 213 are not formed, the concentration of the impurity in the high resistance drift layer 202 can be set to be high even at the same breakdown voltage, and the drift resistance can be lowered. And with this structure, a vertical type MOSFET whose breakdown voltage is larger than 500V can be obtained.
As a second conventional technology, Japanese Patent Application Laid-Open No. 2000-260982 discloses a semiconductor device and a manufacturing method thereof. FIG. 3 is a sectional view showing the structure of the vertical type MOSFET at the second conventional technology. In FIG. 3, each of the parts being equivalent to one of the parts shown in FIG. 1 has the same reference number, therefore the same explanation is omitted. This vertical type MOSFET has planar type gate electrodes and the gate electrodes 206 are not the trench type gate electrodes.
As shown in FIG. 3, a trench 214 is formed in the Nxe2x88x92type epitaxial layer 202 being the high resistance drift layer in the vertical direction, and a Pxe2x88x92type epitaxial layer 215, which connects to the P type base layer 203, is grown in the trench 214. With this, a PN junction being in parallel to the high resistance drift layer 202 is formed. By this structure, at the second conventional technology, as the same as at the first conventional technology, the concentration of the impurity in the high resistance drift layer 202 can be set to be high and the drift resistance can be lowered by securing a certain value of the breakdown voltage.
However, at the first conventional technology, each of the plural Pxe2x88x92type layers 213 is continuously formed in the thickness direction of the high resistance drift layer 202. In order to form this, after a thin Nxe2x88x92type epitaxial layer 202 was grown, a P type impurity is implanted in this thin Nxe2x88x92type epitaxial layer 202, and this process is repeated in plural times and each of the plural Pxe2x88x92type layers 213 is formed in each of the plural thin Nxe2x88x92type epitaxial layers 202. That is, a layered structure is required. After this, the P type impurity is activated by a thermal process, and the high resistance drift layer 202 having a certain thickness is formed. Consequently, the plural Pxe2x88x92type layers 213 can be formed in the vertical direction in a long and deep state, however, there are problems that the number of the processes becomes large and the cost becomes high.
At the second conventional technology, etching for the trench 214 is executed selectively from the surface of the Nxe2x88x92type epitaxial layer 202, and the Pxe2x88x92type epitaxial layer 215 is grown in the trench 214 and embedded. Therefore, technologies to form the deep trench by etching and to grown the epitaxial layer selectively have some difficulties, consequently there is a problem that the cost becomes high.
It is therefore an object of the present invention to provide a vertical type MOSFET and a manufacturing method thereof, in particular, in which its drift resistance is made to be low by securing its breakdown voltage between source and drain electrodes of about 150 V being the middle class breakdown voltage and its manufacturing method is easy and its manufacturing cost is low.
According to a first aspect of the present invention, for achieving the object mentioned above, there is provided a vertical type MOSFET. The vertical type MOSFET provides a high resistance drift layer being a conductivity type on a substrate being a conductivity type and base layers being an opposite conductivity type in designated regions of the surface of the high resistance drift layer and source layers being a conductivity type in the base layers and gate electrodes on specified regions of the surface of the high resistance drift layer. Further the vertical type MOSFET provides a trench type back gate section, which was formed in a first trench positioned at a region between the gate electrodes by filling an insulation material in the first trench, and an impurity layer being an opposite conductivity type, which was formed in the high resistance drift layer at a region right under the trench type back gate section.
According to a second aspect of the present invention, in the first aspect, the impurity layer being the opposite conductivity type consists of plural impurity layers that are positioned in different depths and are connected in the depth direction.
According to a third aspect of the present invention, in the first aspect, the impurity layer being the opposite conductivity type consists of plural impurity layers that are positioned in different depths and are separated in the depth direction.
According to a fourth aspect of the present invention, in the first aspect, each of the gate electrodes is a trench type gate electrode in which a gate insulation film was formed on a second trench formed through one of the source layers and one of the base layers and a conductivity material was filled in the second trench on the gate insulation film.
According to a fifth aspect of the present invention, in the first aspect, each of the gate electrodes is a planar type gate electrode in which a gate insulation film was formed on one of the source layers and a part of one of the base layers and a part of the high resistance drift layer and a conductivity material was formed on the gate insulation film.
According to a sixth aspect of the present invention, in the first aspect, a dielectric interlayer is formed on the source layers and the gate electrodes and the trench type back gate section, and a source electrode connecting to one of the source layers and one of the base layers is formed through a contact hole opened in the dielectric interlayer.
According to a seventh aspect of the present invention, in the sixth aspect, a conductive material being different from a conductive material of the source electrode is filled in the contact hole.
According to an eighth aspect of the present invention, for achieving the object mentioned above, there is provided a manufacturing method of a vertical type MOSFET. The manufacturing method of the vertical type MOSFET provides the steps of, forming a high resistance drift layer being a conductivity type on a substrate being a conductivity type, forming base layers being an opposite conductivity type in designated regions of the surface of the high resistance drift layer, forming source layers being a conductivity type in the base layers, forming gate electrodes in specified regions of the source layers and the base layers reaching to the high resistance drift layer via an insulation film. Further, the manufacturing method of the vertical type MOSFET provides the steps of, forming a trench at a designated position between the gate electrodes in the high resistance drift layer, forming impurity layers being an opposite conductivity type in the high resistance drift layer at the right under position of the trench by implanting ions of an impurity being an opposite conductivity type from the bottom of the trench, and filling an insulation material in the trench.
According to a ninth aspect of the present invention, in the eighth aspect, each of the impurity layers is formed in respective different depth in the high resistance drift layer by plural ion implantation.